* here the difference is even more marked - different signs for the
covariance of the intercept and slope. While not statistically
significant in this example, in the data I am working with there is
significant positive covariance: 0.38 (0.26 - 0.51) from -sem- but a
negative significant covariance from -xtmixed- : -0.18 (-0.32 -0.04)